Widely available are power conversion apparatuses (a converter and an inverter, for example) which obtain a regulated voltage or a regulated current by intermittently supplying power from a power source to an inductor. Such a power conversion apparatus typically employs a free-wheeling diode. A free-wheeling diode rectifies a regenerative current obtained by: a voltage generating coil provided in the power conversion apparatus; and an inductor which is a load coil provided out of the power conversion apparatus.
FIG. 11 is a circuit diagram exemplifying a conventional boost converter.
A boost converter 100 shown in FIG. 11 includes a low-side metal oxide semiconductor field effect transistor (MOSFET) 13, and a high-side MOSFET 12. The MOSFETs 12 and 13 are periodically and exclusively turned on by a gate signal provided from a controller 3.
The MOSFET 12 and the MOSFET 13 respectively include a body diode 12a and a body diode 13a which are parasitic in parallel between a drain and a source.
The MOSFET 13 intermittently supplies power from a power source 14 to an inductor 16 in response to the gate signal provided from the controller 3. This intermittent power supply adds a voltage of the power source 14 to a voltage generated by a counter electromotive force of the inductor 16 to generate an output voltage. The output voltage is supplied to a load 15.
The body diode 12a included in the MOSFET 12 serves as a free-wheeling diode in order to rectify the regenerative current of the inductor 16 (a current supplied to the load 15 in this example).
In general, however, a forward voltage characteristic of a body diode included in a MOSFET is poor. Specifically, an active region having a low current level sees on-resistance of the body diode higher than that of the MOSFET. Thus, as described above, the MOSFET 12 is turned on while the MOSFET 13 is off, and the regenerative current generated by the inductor 16 is supplied to a channel of the MOSFET 12. This technique reduces conduction loss caused at the MOSFET 12. Such a technique, referred to as synchronous rectification, has been widely utilized.
FIG. 12 exemplifies characteristic curves where the gate voltages (Vg) are 0V, +5V, and +10V. Here, each of the characteristic curves represents relationship between a drain current (Id) and a drain-source voltage (Vds) observed at a typical power MOSFET (hereinafter referred to as an Si-MOSFET). The MOSFET is made of silicon (Si) and includes a body diode.
When Vg is 0V, the channel shows no conduction, and a drain current flows into only the body diode. This results in describing a characteristic curve of the body diode. When Vg is +10V or greater, the channel is completely conductive, and the drain current flows into only the channel. This results in describing a characteristic curve of the channel. When Vg is +5V, described is a characteristic curve intermediate between the characteristic curve of the body diode and the characteristic curve of the channel.
Such characteristics of the Si-MOSFET have been commonly known. In the case of a power conversion apparatus handling a practical amount of a drain current (2A to 3A or below in characteristic shown in the graph in FIG. 12, for example) provided under a constraint such as heat release of an element, the power conversion apparatus in general synchronously rectifies the drain current and supplies the drain current to the channel. Compared with the technique of supplying the drain current to the body diode, this technique drops a drain-source voltage, and reduces the conduction loss observed at the Si-MOSFET.
Another technique prohibits the synchronous rectification in the case where the amount of a drain current is so small that drive loss exceeds drain-source conduction loss, the drive loss which represents power used for controlling synchronous rectification including transmission. This technique makes sure to reduce the overall loss of a power conversion apparatus and improves power conversion efficiency (See Patent Reference 1, for example).
Silicon carbide is an excellent semiconductor material applied to a power conversion apparatus. Compared with an Si-MOSFET, an MOSFET made of SiC (referred to as SiC-MOSFET, hereinafter) has preferable characteristics, such as high in rated voltage, quick in response speed, and stable in an operation under a high temperature.
The voltage drop occurring in a pn junction of SiC is approximately 2.5V. This is greater than the voltage drop occurring in that of Si; that is, 0.6V. Hence, when the drain current flows into only the body diode in the SiC-MOSFET, at least a voltage drop of 2.5V occurs between the drain and the source. This conduction loss is greater than a similar case occurred in the Si-MOSFET.
Thus, the synchronous rectification is more important for the power conversion apparatus using the SiC-MOSFET than for the power conversion apparatus using the Si-MOSFET in order to achieve excellent power conversion efficiency.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2000-23456.